(a) Field of the Invention
The present invention relates to a method of manufacturing a picture tube shadow mask and, more particularly, to a method of manufacturing a shadow mask of an Fe--Ni alloy.
(b) Description of the Prior Art
A high-purity, low-carbon steel plate of rimmed steel or aluminum killed steel has been used for a color picture tube shadow mask. The use of this material was determined in consideraion of material supply capacity, manufacturing cost, machining properties, and mechanical strength. However, such a conventional material has a large thermal expansion coefficient (about 12.times.10.sup.-6 /.degree. C. for 0.degree. to 100.degree. C.). The electron beam transmittance of a conventional shadow mask is about 15 to 20%, and many electron beams impinge thereupon, so that the shadow mask itself is heated to a temperature of 30.degree. to 80.degree. C. As a result, the shadow mask is thermally deformed, changing the radius of curvature thereof with respect to a phosphor screen, thereby degrading color purity. Such degradation is called a purity drift (PD). In a conventional color picture tube having a large mask aperture pitch, a wide margin (to be referred to as a guard band quantity hereinafter) for a positional error between the phosphor screen and the electron beam is guaranteed. Even if the shadow mask is thermally deformed to some extent, degradation of color impurity tends not to occur. However, in a high resolution color picture tube used in a character and graphic display unit or a general commercial picture tube having a flat faceplate and a small pitch compatible with character broadcast, the above-mentioned margin is not always sufficient. More specifically, in a high resolution color picture tube, since the aperture pitch is very small, the aperture size itself is also small (140 .mu.m at a pitch of 0.3 mm, or 85 .mu.m at a pitch of 0.2 mm). The guard band quantity is inevitably small. In addition, in order to obtain such a small aperture size by photoetching, the mask plate must have a small thickness, thereby decreasing the heat capacity. As compared with a thick plate, the thermal expansion quantity of such a thin mask plate is increased under identical conditions, thereby degrading the color purity.
In the flat faceplate color picture tube, the radius of the curvature of the mask is larger than that of a normal color picture tube. Even if the mask of the flat tube is subjected to the same thermal expansion influence as in the normal tube, the electron beams passing through the mask apertures are greatly deviated from the target positions on the phosphor screen. In addition to this disadvantage, since the pitch is small, the guard band quantity is small, and the color purity tends to be degraded. In order to resolve the above problems, various methods have been proposed. For example, in Japanese Patent Publication No. 42-25446 and Japanese Patent Disclosure Nos. 50-58977 and 50-68650, an iron-nickel alloy having a small thermal expansion coefficient, e.g., a 36% Ni--Fe Invar alloy (having a thermal expansion coefficient of about 0 to 2.0.times.10.sup.-6 /.degree. C. for 0.degree. to 100.degree. C.) or a 42% Ni--Fe alloy (having a thermal expansion coefficient of about 5.0.times.10.sup.-6 .degree. C. for 0.degree. to 100.degree. C.) is used as a material for a shadow mask. However, such a material cannot satisfy practical application conditions. This is partially because the Invar material of Fe--Ni alloy has poor etching and molding properties as compared with those of the conventional low-carbon steel plate. With respect to the etching method, various proposals have been made as exemplified in Japanese Patent Publication No. 59-32859 and Japanese Patent Disclosure No. 59-149638. The shadow mask must have a surface curved with high precision. Tolerance for the radius R of curvature of 1,000 mm is as strict as .+-.5 mm. However, as compared with the iron-based alloy, the Fe--Ni alloy has a high mechanical strength and a poor spherical formability by pressing or the like even after annealing under the same conditiohs. For example, as shown in FIG. 1, when an Fe--Ni mask having a thickness of 0.2 mm is formed spherically and a local recess is formed with respect to a standard radius R of curvature, a depth d of the recess which is not more than 20 .mu.m substantially satisfies the tolerance requirement for color purity. FIG. 2 is a graph showing the recess depth as a function of the yield strength in a 14 inch type shadow mask. As apparent from FIG. 2, the yield strength must be less than 20kg/mm.sup.2 so as to limit the depth to 20 .mu.m or less. When a shadow mask of an Fe--Ni alloy material is annealed in an annealing furnace in a hydrogen atmosphere provided for the conventional shadow mask of aluminum killed steel material, the yield strength (a curve b) of the Fe--Ni alloy is higher than the yield strength (a curve a) of the aluminum killed low-carbon steel, as shown in FIG. 3. The yield strength of the Fe--Ni alloy is decreased only to 29 to 30 kg/m.sup.-2 even if it is annealed at a high temperature of 900.degree. C. Referring to FIG. 2, the yield strength of the Fe--Ni alloy does not show a yield phenomenon inherent to carbon steel and is represented by the tensile strength when the Fe--Ni alloy is elongated by 0.2%. In this manner, the effective peripheral portion of the shadow mask of the Fe--Ni alloy material is particularly subject to deformation and recessing, thereby presenting the problem of degradation in color purity due to deformation. While in order to improve anticorrosion and heat radiation properties of the shadow mask to be incorporated in a tube, a desired curved surface is obtained by pressing and a darkened oxide layer (to be referred to as a darkened layer hereinafter) is formed on the surface of the shadow mask. Although it has been assumed that the darkened layer need not be formed on the Fe--Ni shadow mask due to the presence of Ni having good anticorrosion properties, a typical difference between the electron beam mobility (i.e., PD quantity) of the Fe--Ni alloy without the darkened layer caused by thermal deformation thereof and that of the aluminum killed low carbon steel cannot be observed even if the Fe--Ni has a small thermal expansion coefficient. This is because heat radiation is degraded since the darkened layer is not formed on the shadow mask, and a thermal conductivity of the Fe--Ni alloy is lower than that of the aluminum killed low-carbon steel. Thus, under identical operating conditions, the Fe--Ni shadow mask has a higher temperature than that of the low-carbon steel shadow mask. Therefore, unless the darkened layer having good heat radiation is formed on the shadow mask, the low thermal expansion of the Fe--Ni material cannot be effectively utilized, resulting in degradation of color purity caused by thermal deformation. However, it is very difficult to form a dense, darkened layer on the Fe--Ni alloy with good adhesion by means of a conventional method since Fe--Ni alloy has good anticorrosion properties. The darkened layer tends to be nonuniform due to impurities contained in the Fe--Ni alloy or surface contamination of the Fe--Ni mask. Consequently, a red rust is partially formed on the surface of the Fe--Ni mask. Furthermore, during formation of the darkened layer, a stress acts on the coarse inner wall surface of each shadow mask aperture due to the difference between the thermal expansion coefficients of the darkened layer and the shadow mask material. In a worst case, the darkened layer peels from the surface of the Fe--Ni alloy. Rust increases in the area of red rust formation during subsequent heat treatment to vary the aperture sizes. As compared with the darkened layer, the red rust layer more easily peels from the Fe--Ni material. The peeled, darkened and red rust layers cause a decrease in breakdown voltage, resulting in a notable disadvantage to the color picture tube.